This application claims priority of German patent application no. 101 03 097.5, filed Jan. 24, 2001, the complete disclosure of which is hereby incorporated by reference.
The invention relates to a device for generating a quadrangular illuminating field, having a light mixing rod, which has a quadrangular cross-section as well as a quadrangular inlet area and a quadrangular outlet area and guides light coupled in via the inlet area to the outlet area, in order to generate therein the quadrangular illuminating field, said outlet area being limited by four rectilinear sides, of which two each converge in one of the corners of the outlet area. The invention further relates to the use of such device for generating a quadrangular illuminating field in an optical device comprising a surface to be illuminated, which has a predetermined shape, and illumination optics, which form an image of the outlet area on the surface to be illuminated.
Such device for generating a quadrangular illuminating field is employed, for example, in the illumination optics of a digital projector, as it is called, which also comprises, in addition to the illumination optics, a generally rectangular imaging element, such as a tilting mirror matrix or an LCD matrix, and projection optics for projecting the image generated by the imaging element. In such a digital projector, it is required for high-quality images that the imaging element be illuminated as uniformly as possible by the illumination optics. Therefore, the illumination optics contain the light mixing rod, whose outlet area conventionally has a rectangular shape, if the imaging element is rectangular. It has been found, however, that a complicated optical system is necessary for forming an image of the illuminating field in the outlet area of the light mixing rod on the imaging element, so as to ensure that the image is formed in a manner as free from distortion as possible. In many cases, there also arises the additional problem that the outlet area and the surface of the imaging element are not situated in planes which are parallel to each other, but are inclined relative to each other, which also causes distortions leading to decreased uniformity of the illumination of the imaging element.
Therefore, it is an object of the invention to improve the above-described type of device for generating a quadrangular illuminating field such that it allows illuminating, as uniformly as possible, a surface which is to be illuminated and has a predetermined shape.
According to the invention, this object is achieved, in connection with the above-described type of device for generating a quadrangular illuminating field, in that the two sides respectively meeting in one corner of the outlet area do not converge at a right angle. Preferably, all four interior angles have values differing from each other in the corners. Thus, the shape of the outlet area of the light mixing rod may be selected such that the distortion caused by forming an image of the outlet area on the surface to be illuminated is compensated. The surface to be illuminated will then be uniformly illuminated. The light mixing rod may preferably extend rectilinearly or at an angle.
In a preferred further embodiment of the device according to the invention, the light mixing rod is formed such that each of its cross-sectional areas is limited by four rectilinear sides, which converge at the same angles as the sides of the outlet area. This configuration allows the light mixing rod to be produced in a simple manner and with the required accuracy. Thus, for example, it may be formed as a solid mixing rod by grinding a blank, or as a hollow mixing rod with four reflectively coated plates.
Moreover, the device according to the invention may be further embodied such that the cross-sectional area of the light mixing rod decreases from the inlet area to the outlet area. Since, in a light mixing rod, the product of the inlet area with the aperture of the inlet area equals the product of the outlet area with the aperture of the outlet area, a larger aperture is possible at the outlet area. At the same time, the inlet area may be large, which facilitates the coupling-in of the light in the light mixing rod.
A particularly preferred further embodiment of the device according to the invention consists in that the light mixing rod is made of a light-transparent material and is, consequently, a solid mixing rod, in which the light which does not travel directly from the inlet area to the outlet area is guided by total reflexion. Since no losses occur in said total reflexion and, consequently, only the losses caused by material absorption are present, nearly all of the coupled-in light is guided to the outlet area, so that a very high efficiency in light transmission is realized. Moreover, it is also an advantage that excessive heating of the light mixing rod is prevented because total reflexion involves no losses which might contribute to such heating.
A particular further embodiment of the device according to the invention consists in that the light mixing rod has a hollow cross-section, which is formed by four reflective surfaces extending from the inlet area to the outlet area. This configuration of the light mixing rod allows simple and accurate production. A further particular advantage of this embodiment is that the outlet area is not formed by an end surface of the material, from which the light mixing rod is produced, but is the region at the end of the light mixing rod between the four reflective surfaces, thus usually an air portion, since the light mixing rod is conventionally present in a surrounding atmosphere and thus in air. This leads to the advantage that no soiling or misting, which might adversely affect the uniformity of the generated quadrangular illuminating field, can form in the outlet area.
In particular, the device according to the invention may be further embodied such that the light mixing rod comprises a solid mixing rod portion made of a light-transparent material and an end portion optically coupled therewith, which is provided with reflective surfaces and has a hollow cross-section and whose end averted from the solid mixing rod portion forms the outlet or inlet area. As the outlet or inlet area is formed by the end of the end portion averted from the solid mixing rod portion, this prevents that soiling or misting may accumulate on this surface, so that the illuminating field generated in the outlet area is not adversely affected.
An advantageous embodiment of the device according to the invention consists in that the light mixing rod comprises a first and second light guiding region for separately guiding components of the coupled-in light due to a partition extending from the inlet area to the outlet area. Thus, it is possible to create an intentional non-uniformity of the luminance in the illuminating field. This non-uniformity may be adapted to uniformly illuminate the surface to be illuminated.
The device according to the invention may be used, in particular, in an optical device comprising a surface to be illuminated, which has a predetermined shape, and illumination optics, which form an image of the outlet area on the surface to be illuminated. Preferably, the optical device also comprises projection optics for projecting the surface to be illuminated onto a projection surface. Thus, an optical device is provided wherein a more uniform illumination of the surface to be illuminated is possible due to the device according to the invention, because the distortions caused by the illumination optics may be compensated by suitably selecting the shape of the quadrangular outlet area of the light mixing rod.